ipv6: icmp6: add drop reason support to ndisc_router_discovery()

Change ndisc_router_discovery() to return a drop reason.

For the moment, return PKT_TOO_SMALL, NOT_SPECIFIED
and SKB_CONSUMED.

More reasons are added later.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>